Conventional mechanical and electro-mechanical power switches suffer from disadvantages including high levels of noise and spike voltages generated when an electrical load is switched on. To overcome these problems, a solid state power controller (SSPC) may be utilized, which includes a solid state switching device, e.g., a power MOSFET switch. Such SSPCs also have the advantages of occupying less space and being light-weight.
In an electrical system with a variable frequency AC power source, it is desirable to switch-on the loads (e.g., single-phase and three-phase loads) at zero-crossing points of the waveform of the voltage generated by the power source. This reduces the harmonics that are produced when the loads are turned on, and minimizes electromagnetic interference (EMI) effects. It is also advantageous to switch-off such loads at the zero-crossing points of the current supplied by the power source, in order to reduce the effects of EMI and thermal stress on the switching devices and load equipment.